Blank cards are widely used for electromagnetic interference (EMI) containment and airflow distribution purposes in any electronic enclosures, such as enterprise switches or computing servers. However the perforation on the blank cards is typically designed to have small openings for better EMI shielding and higher airflow impedance to prevent airflow bypass.
Theoretically, the concept of high impedance blanks makes sense because it would force more airflow distribution to the functional cards within a system, hence a better cooling scheme. However, with high impedance blanks in the system, this also translates to high system resistance, hence a lower overall system airflow. The result is actually a hotter line-card or blade due to the reduction of total system airflow.
To make matters worse, there are various line-cards that can go into a same system enclosure, where each of the line-cards has a very different airflow impedance (e.g., due to the density of front end I/O connectors, perforations, general design, etc.), thus, a single impedance blank design would soon be a problem when high-power and high-impedance card are inserted in the system. The combination of high-impedance blanks and functional cards would further lower the system airflow throughput, thus causing over-heating of the line-card(s).